Oxygen is a crucial nutrient for human cells. Cell damage may result from oxygen deprivation for even brief periods of time, which may lead to organ dysfunction or failure. For example, heart attack and stroke victims experience blood flow obstructions or diversions that prevent oxygen from being delivered to the cells of vital tissues. Without oxygen, the heart and brain progressively deteriorate. In severe cases death results from complete organ failure. Less severe cases typically involve costly hospitalization, specialized treatments and lengthy rehabilitation.
Blood oxygen levels may be described in terms of the concentration of oxygen that would be achieved in a saturated solution at a given partial pressure of oxygen (pO2). Typically, for arterial blood, normal blood oxygen levels (i.e., normoxia or normoxemia) range from 90–110 mm Hg. Hypoxemic blood (i.e., hypoxemia) is arterial blood with a pO2 less than 90 mm Hg. Hyperoxic blood (i.e., hyperoxemia or hyperoxia) is arterial blood with a pO2 greater than 400 mm Hg (see Cason et. al (1992) Effects of High Arterial Oxygen Tension on Function, Blood Flow Distribution, and Metabolism in Ischemic Myocardium, Circulation, 85(2):828–38, but less than 760 mm Hg (see Shandling et al. (1997) Hyperbaric Oxygen and Thrombolysis in Myocardial Infarction: The “HOT MI” Pilot Study, American Heart Journal 134(3):544–50). Hyperbaric blood is arterial blood with a pO2 greater than 760 mm Hg. Venous blood typically has a pO2 level less than 90 mm Hg. In the average adult, for example, normal venous blood oxygen levels range generally from 40 mm Hg to 70 mm Hg.
Blood oxygen levels also might be described in terms of hemoglobin saturation levels. For normal arterial blood, hemoglobin saturation is about 97% and varies only slightly as pO2 levels increase. For normal venous blood, hemoglobin saturation is about 75%.
In patients who suffer from acute myocardial infarction, if the myocardium is deprived of adequate levels of oxygenated blood for a prolonged period of time, irreversible damage to the heart can result. Where the infarction is manifested in a heart attack, the coronary arteries fail to provide adequate blood flow to the heart muscle.
Treatment of acute myocardial infarction or myocardial ischemia often comprises performing angioplasty or stenting of the vessels to compress, ablate or otherwise treat the occlusion(s) within the vessel walls. For example, a successful angioplasty uses a balloon to increase the size of the vessel opening to allow increased blood flow.
Even with the successful treatment of occluded vessels, a risk of tissue injury may still exist. During percutaneous transluminal coronary angioplasty (PTCA), the balloon inflation time is limited by the patient's tolerance to ischemia caused by the temporary blockage of blood flow through a vessel during balloon inflation. Reperfusion injury also may result, for example, due to slow coronary reflow or no reflow following angioplasty.
For some patients angioplasty procedures are not an attractive option for the treatment of vessel blockages. Such patients typically are at increased risk of ischemia for reasons such as poor left ventricular function, lesion type and location, or the amount of the myocardium at risk. The treatment options for such patients thus include more invasive procedures such as coronary bypass surgery.
To reduce the risk of tissue injury typically associated with treatments of acute myocardial infarction and myocardial ischemia, it is usually desirable to deliver oxygenated blood or oxygen-enriched fluids to at-risk tissues. Tissue injury is minimized or prevented by the diffusion of the dissolved oxygen from the blood or fluids to the tissue and/or blood perfusion that removes metabolites and that provides other chemical nutrients.
In some cases, the desired treatment of acute myocardial infarction and myocardial ischemia includes perfusion of oxygenated blood or oxygen-enriched fluids. During PTCA, for example, tolerated balloon inflation time may be increased by the concurrent introduction of oxygenated blood into the patient's coronary artery. Increased blood oxygen levels also may cause the normally perfused left ventricular cardiac tissue into hypercontractility to further increase blood flow through the treated coronary vessels.
The infusion of oxygenated blood or oxygen-enriched fluids also may be continued following the completion of PTCA treatment or other procedures (e.g. surgery) wherein cardiac tissue “stunning” with associated function compromise has occurred. In some cases continued infusion may accelerate the reversal of ischemia and facilitate recovery of myocardial function.
Conventional methods for the delivery of oxygenated blood or oxygen-enriched fluids to at-risk tissues involve the use of blood oxygenators. Such procedures generally involve withdrawing blood from a patient, circulating it through an oxygenator to increase blood oxygen concentration, and then delivering the blood back to the patient. One example of a commercially available blood oxygenator is the Maxima blood oxygenator manufactured by Medtronic, Inc., Minneapolis, Minn.
There are drawbacks, however, to the use of a conventional oxygenator in an extracorporeal circuit for oxygenating blood. Such systems typically are costly, complex and difficult to operate. Often a qualified perfusionist is required to prepare and monitor the system.
Conventional oxygenator systems also typically have a large priming volume, i.e., the total volume of blood contained within the oxygenator, tubing and other system components, and associated devices. It is not uncommon in a typical adult patient case for the oxygenation system to hold more than one to two liters of blood. Such large priming volumes are undesirable for many reasons. For example, in some cases a blood transfusion may be necessary to compensate for the blood temporarily lost to the oxygenation system because of its large priming volume. Heaters often must be used to maintain the temperature of the blood at an acceptable level as it travels through the extracorporeal circuit. Further, conventional oxygenator systems are relatively difficult to turn on and off. For instance, if the oxygenator is turned off, large stagnant pools of blood in the oxygenator might coagulate.
In addition, with extracorporeal circuits including conventional blood oxygenators there is a relatively high risk of inflammatory cell reaction and blood coagulation due to the relatively slow blood flow rates and the large blood contact surface area. A blood contact surface area of about 1–2 m2 and velocity flows of about 3 cm/s are not uncommon with conventional oxygenator systems. Thus, relatively aggressive anti-coagulation therapy, such as heparinization, is usually required as an adjunct to using the oxygenator.
Perhaps one of the greatest disadvantages to using conventional blood oxygenation systems is that the maximum partial pressure of oxygen (pO2) that can be imparted to blood with commercially available oxygenators is about 500 mm Hg. Thus, blood pO2 levels near or above 760 mm Hg cannot be achieved with conventional oxygenators.
Some experimental studies to treat myocardial infarction have involved the use of hyperbaric oxygen therapy. See, e.g., Shandling et al. (1997), Hyperbaric Oxygen and Thrombolysis in Myocardial Infarction: The “HOT MI” Pilot Study, American Heart Journal 134(3):544–50. These studies generally have involved placing patients in chambers of pure oxygen pressurized at up to 2 atmospheres, resulting in systemic oxygenation of patient blood up to a pO2 level of about 1200 mm Hg. However, use of hyperbaric oxygen therapy following restoration of coronary artery patency in the setting of an acute myocardial infarction is not practical. Monitoring critically ill patients in a hyperbaric oxygen chamber is difficult. Many patients become claustrophobic. Ear damage may occur. Further, treatment times longer than 90 minutes cannot be provided without concern for pulmonary oxygen toxicity.
For these reasons, the treatment of regional organ ischemia generally has not been developed clinically. Thus, there remains a need for a simple and convenient system for delivering oxygenated blood and other fluids to patients for the localized prevention of ischemia and the treatment of post-ischemic tissue and organs.